Ferritic stainless steel and production method therefor (as amended)

ABSTRACT

Provided is a ferritic stainless steel that has excellent corrosion resistance and displays good brazing properties when brazing is carried out at high temperature using a Ni-containing brazing metal. These effects are obtained as a result of the steel having a chemical composition containing, in mass %: 0.003% to 0.020% of C; 0.05% to 1.00% of Si; 0.10% to 0.50% of Mn, 0.05% or less of P; 0.01% or less of S; 16.0% to 25.0% of Cr; 0.05% to 0.35% of Ti; 0.005% to 0.05% of Al; and 0.005% to 0.025% of N, the balance being Fe and incidental impurities, and as a result of a nitrogen-enriched layer being created that has a nitrogen concentration peak value of 0.05 mass % to 0.30 mass % at a depth of within 0.05 μm of a surface of the steel.

TECHNICAL FIELD

The present disclosure relates to a ferritic stainless steel havingexcellent corrosion resistance and displaying good brazing propertieswhen brazing is carried out at high temperature using a Ni-containingbrazing metal, and to a production method for the ferritic stainlesssteel.

BACKGROUND

In recent years, there has been demand for further improvement ofautomobile fuel efficiency and exhaust gas purification from astandpoint of environmental protection. Consequently, adoption ofexhaust heat recovery units and EGR (Exhaust Gas Recirculation) coolersin automobiles continues to increase.

An exhaust heat recovery unit is an apparatus that improves fuelefficiency by, for example, using heat from engine coolant forautomobile heating and using heat from exhaust gas to warm up enginecoolant in order to shorten warming-up time when the engine is startedup. The exhaust heat recovery unit is normally located between acatalytic converter and a muffler, and includes a heat exchanger partformed by a combination of pipes, plates, fins, side plates, and soforth, and entry and exit pipe parts. Exhaust gas enters the heatexchanger part through the entry pipe, transfers its heat to a coolantvia a heat-transfer surface such as a fin, and is discharged from theexit pipe. Bonding and assembly of plates, fins, and so forth formingthe heat exchanger part of an exhaust heat recovery unit such asexplained above is mainly carried out by brazing using a Ni-containingbrazing metal.

An EGR cooler includes a pipe for intake of exhaust gas from an exhaustmanifold or the like, a pipe for returning the exhaust gas to a gasintake-side of an engine, and a heat exchanger for cooling the exhaustgas. The EGR cooler more specifically has a structure in which a heatexchanger including both a water flow passage and an exhaust gas flowpassage is located on a path along which exhaust gas is returned to thegas intake-side of the engine from the exhaust manifold. Through thestructure described above, high-temperature exhaust gas at theexhaust-side is cooled by the heat exchanger and the cooled exhaust gasis returned to the gas intake-side such as to lower the combustiontemperature of the engine. Accordingly, this structure forms a systemfor inhibiting NO_(x) production, which tends to occur at hightemperatures. Furthermore, the heat exchanger part of the EGR cooler isformed by overlapping thin plates in a fin shape for reasons such asimproving compactness, and reducing weight and cost. Bonding andassembly of these thin plates is mainly carried out by brazing using aNi-containing brazing metal.

Since bonding and assembly for a heat exchanger part in an exhaust heatrecovery unit or an EGR cooler such as described above are carried outby brazing using a Ni-containing brazing metal, materials used in theheat exchanger part are expected to have good brazing properties withrespect to the Ni-containing brazing metal. Moreover, a heat exchangerpart such as described above is expected to be highly resistant tooxidation caused by high-temperature exhaust gas passing through theheat exchanger part. The exhaust gas includes small amounts of nitrogenoxides (NO_(x)), sulfur oxides (SO_(x)), and hydrocarbons (HC) that maycondense in the heat exchanger to form a strongly acidic and corrosivecondensate. Therefore, materials used in a heat exchanger part such asdescribed above are expected to have corrosion resistance at normaltemperatures. In particular, because brazing heat treatment is carriedout at high temperature, it is necessary to prevent formation of a Crdepletion layer due to preferential reaction of Cr at grain boundarieswith C and N, which is referred to as sensitization, in order to ensurethat corrosion resistance is obtained.

For the reason described above, heat exchanger parts of exhaust heatrecovery units and EGR coolers are normally made using anaustenite-based stainless steel such as SUS316L or SUS304L that has areduced carbon content and is resistant to sensitization. However,austenite-based stainless steels suffer from problems such as high costdue to having high Ni content, and also poor heat fatigue properties athigh temperatures and poor fatigue properties when used in anenvironment in which constraining force is received at high temperatureand with violent vibration, such as when used as a component locatedperipherally to an exhaust manifold.

Therefore, steels other than austenite-based stainless steels are beingconsidered for use in heat exchanger parts of exhaust heat recoveryunits and EGR coolers.

For example, PTL 1 discloses, as a heat exchanger component of anexhaust heat recovery unit, a ferritic stainless steel that has addedMo, Ti, or Nb and that has reduced Si and Al content. PTL 1 disclosesthat addition of Ti or Nb prevents sensitization by stabilizing C and Nin the steel as carbonitrides of Ti and Nb and that reduction of Si andAl content improves brazing properties.

PTL 2 discloses, as a component for a heat exchanger of an exhaust heatrecovery unit, a ferritic stainless steel having excellent condensatecorrosion resistance in which Mo content is defined by Cr content, andTi and Nb content is defined by C and N content.

Furthermore, PTL 3 discloses, as a material for an EGR cooler, aferritic stainless steel in which added amounts of components such asCr, Cu, Al, and Ti satisfy a certain relationship.

Additionally, PTL 4 and 5 disclose, as a component of an EGR cooler anda material for a heat exchanger part of an EGR cooler, a ferriticstainless steel containing 0.3 mass % to 0.8 mass % of Nb and a ferriticstainless steel containing 0.2 mass % to 0.8 mass % of Nb.

CITATION LIST Patent Literature

PTL 1: JP H7-292446 A

PTL 2: JP 2009-228036 A

PTL 3: JP 2010-121208 A

PTL 4: JP 2009-174040 A

PTL 5: JP 2010-285683 A

PTL 6: JP 2842787 B

SUMMARY Technical Problem

However, there is a presumption that brazing of the steel disclosed inPTL 1 is carried out using a copper brazing metal having a low brazingtemperature and inadequate brazing may, therefore, occur in a situationin which a Ni-containing brazing metal (for example, BNi-2 or BNi-5stipulated by Japanese Industrial Standards (JIS Z 3265)) having a highbrazing temperature is used.

In the case of the steel disclosed in PTL 2, in particular steelcontaining Ti, a problem of reduced brazing properties may occur as aresult of a thick Ti oxide film being formed such that spreading of thebrazing metal is decreased when brazing is carried out at a temperaturethat is high, even among brazing metals in which a Ni-containing brazingmetal is used.

Furthermore, although the chemical composition of the steel disclosed byPTL 3 takes into account inhibition of Ti or Al oxide film formationduring brazing at high temperature using a Ni-containing brazing metal,this inhibitive effect is not thought to be sufficient. Consequently, ithas not necessarily been possible to achieve adequate brazing propertiesdue to, for example, unsatisfactory joint strength or unsatisfactorybrazing metal infiltration into a joint gap between overlapping partswhen overlapping steel is brazed.

In relation to this point, steel disclosed in PTL 4 and 5 has a high Nbcontent in order to inhibit coarsening of crystal grains during brazingusing a Ni-containing brazing metal and prevent reduction in toughness,and a certain degree of improvement of brazing properties is obtained ina situation in which Ti and Al are not contained in the steel.

However, the high Nb content leads to a higher recrystallizationtemperature, which causes growth of a thicker oxide film, referred to asa scale, during final annealing. Consequently, descaling properties in adescaling process performed after the annealing are negatively affected,which is problematic because it makes it difficult to adopt an efficientproduction process (high-speed pickling process) using a normal carbonsteel production line as disclosed in PTL 6. Nb is also expensive, whichis problematic in terms of production costs.

The present disclosure is the result of development conducted in orderto solve the problems described above and an objective thereof is toprovide a ferritic stainless steel that has excellent corrosionresistance, displays good brazing properties when brazing is carried outat high temperature using a Ni-containing brazing metal, and can beproduced by a highly efficient production process, and also to provide aproduction method for this ferritic stainless steel.

Solution to Problem

The inventors decided to use Ti as a stabilizing element for C and N dueto the fact that, unlike Nb addition, Ti addition does not lead to ahigher recrystallization temperature. The inventors conducted diligentinvestigation in which they produced Ti-containing ferritic stainlesssteel using various different chemical compositions and productionconditions, and investigated various properties thereof, particularlybrazing properties when brazing is carried out at high temperature usinga Ni-containing brazing metal.

However, no matter how the chemical composition was adjusted inproduction of the Ti-containing ferritic stainless steel describedabove, it was not possible to satisfactorily inhibit formation of anoxide film of Ti, Al, or the like, which negatively affects spreading ofbrazing metal, during brazing carried out at high temperature using aNi-containing brazing metal. As a result, desired brazingproperties—specifically, brazing metal infiltration into a joint gapbetween overlapping parts when overlapping steel is brazed and brazedpart joint strength—could not be adequately obtained.

Therefore, the inventors conducted further investigation with anobjective of effectively inhibiting formation of an oxide film of Ti,Al, or the like when brazing is carried out at high temperature using aNi-containing brazing metal.

As a result of this investigation, the inventors discovered that it ispossible to prevent formation of an oxide film of Ti, Al, or the likeduring brazing by subjecting the steel to heat treatment in a controlledatmosphere prior to brazing such that a specific nitrogen-enriched layeris formed in a surface layer part of the steel. It was also discoveredthat through formation of this nitrogen-enriched layer, good brazingproperties can be satisfactorily obtained even when brazing is carriedout at high temperature using a Ni-containing brazing metal.

The inventors also realized that steel having a nitrogen-enriched layerformed therein as described above is also extremely advantageous interms of production efficiency because an efficient production processis applicable thereto.

Based on these findings, the inventors conducted further investigationwhich eventually led to the present disclosure.

Specifically, the primary features of the present disclosure are asfollows.

1. A ferritic stainless steel comprising

a chemical composition containing (consisting of), in mass %:

0.003% to 0.020% of C;

0.05% to 1.00% of Si;

0.10% to 0.50% of Mn;

0.05% or less of P;

0.01% or less of S;

16.0% to 25.0% of Cr;

0.05% to 0.35% of Ti;

0.005% to 0.05% of Al; and

0.005% to 0.025% of N,

the balance being Fe and incidental impurities, wherein

a nitrogen-enriched layer is present that has a nitrogen concentrationpeak value of 0.05 mass % to 0.30 mass % at a depth of within 0.05 μm ofa surface of the steel.

2. The ferritic stainless steel described above in 1, wherein

the chemical composition further contains, in mass %, one or more of:

0.05% to 0.50% of Ni;

0.10% to 3.00% of Mo;

0.10% to 0.60% of Cu;

0.01% to 0.50% of V;

0.01% to 0.15% of Nb;

0.0003% to 0.0040% of Ca; and

0.0003% to 0.0100% of B.

3. A production method for the ferritic stainless steel described abovein 1 or 2, comprising

subjecting a slab having the chemical composition described above in 1or 2 to hot rolling, subsequent hot band annealing as required, and asubsequent combination of cold rolling and annealing to produce theferritic stainless steel, wherein

in final annealing of the annealing, treatment for creating anitrogen-enriched layer is performed at a temperature of 800° C. orhigher in an atmosphere having a dew point of −20° C. or lower and anitrogen concentration of 5 vol % or greater.

Advantageous Effect

According to the present disclosure, a ferritic stainless steel can beobtained that has excellent corrosion resistance and that displays goodbrazing properties when brazing is carried out at high temperature usinga Ni-containing brazing metal.

Moreover, the presently disclosed ferritic stainless steel can beproduced by a highly efficient production process and is, therefore,extremely advantageous in terms of productions costs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view illustrating a test material used toevaluated joint gap infiltration by a brazing metal; and

FIG. 2 schematically illustrates a tensile test piece used to evaluatejoint strength of a brazed part, wherein FIG. 2A illustrates one side ofthe tensile test piece prior to brazing and FIG. 2B illustrates theentire tensile test piece after brazing.

DETAILED DESCRIPTION

The following provides a specific description of the present disclosure.

First, the reasons for limiting the chemical composition of the steel tothe aforementioned range in the present disclosure are explained.Hereinafter, the unit “%” relating to the content of elements in thechemical composition of the steel refers to “mass %” unless specifiedotherwise.

C: 0.003% to 0.020%

C is an element contained incidentally in the steel. Strength of thesteel improves with increasing C content whereas workability of thesteel improves with decreasing C content. Herein, the C content isrequired to be 0.003% or greater in order to obtain sufficient strength.However, if the C content is greater than 0.020%, workability noticeablydecreases and sensitization tends to occur more easily due to Cr carbideprecipitation at grain boundaries. Accordingly, the C content is in arange of 0.003% to 0.020%. Furthermore, although low C content ispreferable from a viewpoint of corrosion resistance, if the C content isset too low, refining becomes time consuming, leading to increasedcosts. Accordingly, the C content is preferably in a range of 0.010% to0.020%.

Si: 0.05% to 1.00%

Si is a useful element as a deoxidizer. This effect is obtained throughSi content of 0.05% or greater. However, if Si content is greater than1.00%, workability noticeably decreases and forming becomes difficult.Furthermore, application of a high-speed pickling process using a normalcarbon steel production line as described in PTL 6 becomes difficult ifthe Si content is greater than 1.00%. Accordingly, the Si content is ina range of 0.05% to 1.00%. The Si content is preferably in a range of0.10% to 0.50%. Moreover, an upper limit for the Si content is morepreferably 0.40%, and particularly preferably 0.30%.

Mn: 0.10% to 0.50%

Mn has a deoxidizing effect that is obtained through Mn content of 0.10%or greater. However, excessive Mn addition leads to loss of workabilitydue to solid solution strengthening. Furthermore, excessive Mn decreasescorrosion resistance by promoting precipitation of MnS, which acts as astarting point for corrosion. Therefore, Mn content of 0.50% or less isappropriate. Accordingly, the Mn content is in a range of 0.10% to0.50%. The Mn content is preferably in a range of 0.15% to 0.50%.Moreover, an upper limit for the Mn content is more preferably 0.35%,and particularly preferably 0.25%.

P: 0.05% or less

P is an element that is incidentally included in the steel. However,excessive P content reduces weldability and facilitates grain boundarycorrosion. This trend is noticeable if the P content is greater than0.05%. Accordingly, the P content is 0.05% or less. The P content ispreferably 0.03% or less.

However, since excessive dephosphorization leads to increased refiningtime and costs, the P content is preferably 0.02% or greater.

S: 0.01% or less

S is an element that is incidentally contained in the steel, and thatpromotes MnS precipitation and decreases corrosion resistance if Scontent is greater than 0.01%. Accordingly, the S content is 0.01% orless. The S content is preferably 0.007% or less.

Cr: 16.0% to 25.0%

Cr is an important element for ensuring corrosion resistance of thestainless steel. Adequate corrosion resistance after brazing is notobtained if Cr content is less than 16.0%. However, excessive additionof Cr causes deterioration of workability. Accordingly, the Cr contentis in a range of 16.0% to 25.0%. The Cr content is preferably in a rangeof 18.0% to 23.0%.

Ti: 0.05% to 0.35%

Ti is an element that prevents the precipitation of Cr carbonitride,which decreases corrosion resistance (sensitization), since Ti combineswith C and N preferentially. This effect is obtained through Ti contentof 0.05% or greater. However, Ti is not a particularly preferableelement from a viewpoint of brazing properties. The reason for this isthat Ti is an active element with respect to oxygen and thus brazingproperties are decreased as a result of a dense and continuous Ti oxidefilm being formed during brazing. Ti oxide film formation is preventedin the present disclosure through creation of a nitrogen-enriched layerin a surface layer of the steel, but it is not possible to adequatelyprevent Ti oxide film formation if Ti content is greater than 0.35%.Accordingly, the Ti content is in a range of 0.05% to 0.35%. The Ticontent is preferably in a range of 0.10% to 0.25%, and is morepreferably in a range of 0.10% to 0.20%.

Al: 0.005% to 0.05%

Al is a useful element for deoxidization, which is obtained as an effectthrough Al content of 0.005% or greater. However, in the same way as Ti,Al is not a particularly preferable element from a viewpoint of brazingproperties. The reason for this is that, in the same way as Ti, Alcauses formation of a dense and continuous Al oxide film (Al₂O₃ film) atthe surface of the steel during brazing and therefore negatively affectsbrazing properties as a result of the Al oxide film hindering spreadingand adhesion of the brazing metal. Al oxide film formation is preventedin the present disclosure through creation of the nitrogen-enrichedlayer in the surface layer of the steel, but it is not possible toadequately prevent Al oxide film formation if Al content is greater than0.05%. Accordingly, the Al content is in a range of 0.005% to 0.05%. TheAl content is preferably in a range of 0.01% to 0.03%.

N: 0.005% to 0.025%

N is an important element for preventing Ti or Al oxide film formationand improving brazing properties through creation of thenitrogen-enriched layer. N content is required to be 0.005% or greaterin order to create the nitrogen-enriched layer. However, N content ofgreater than 0.025% facilitates sensitization and reduces workability.Accordingly, the N content is in a range of 0.005% to 0.025%. The Ncontent is preferably in a range of 0.007% to 0.020%.

In addition to the basic components described above, the chemicalcomposition in the present disclosure may appropriately further containthe following elements as required.

Ni: 0.05% to 0.50%

Ni is an element that effectively contributes to improving toughness andto improving crevice corrosion resistance when contained in an amount of0.05% or greater. However, Ni content of greater than 0.50% increasesstress corrosion crack sensitivity. Furthermore, Ni is an expensiveelement that leads to increased costs. Accordingly, in a situation inwhich Ni is contained in the steel, the Ni content is in a range of0.05% to 0.50%. The Ni content is preferably in a range of 0.10% to0.30%.

Mo: 0.10% to 3.00%

Mo improves corrosion resistance by stabilizing a passivation film ofthe stainless steel. In the case of an exhaust heat recovery unit or anEGR cooler, Mo has an effect of preventing inner surface corrosion by acondensate and outer surface corrosion by a snow-melting agent or thelike. Furthermore, Mo has an effect of improving high-temperature heatfatigue properties and is a particularly preferable element in asituation in which the steel is used in an EGR cooler attached directlybelow an exhaust manifold. These effects are obtained through Mo contentof 0.10% or greater. However, Mo is an expensive element that leads toincreased costs. Furthermore, Mo content of greater than 3.00% reducesworkability. Accordingly, in a situation in which Mo is contained in thesteel, the Mo content is in a range of 0.10% to 3.00%. The Mo content ispreferably in a range of 0.50% to 2.50%.

Cu: 0.10% to 0.60%

Cu is an element that enhances corrosion resistance. This effect isobtained through Cu content of 0.10% or greater. However, Cu content ofgreater than 0.60% reduces hot workability. Accordingly, in a situationin which Cu is contained in the steel, the Cu content is in a range of0.10% to 0.60%. The Cu content is preferably in a range of 0.20% to0.50%.

V: 0.01% to 0.50%

V combines with C and N contained in the steel and preventssensitization in the same way as Ti. V also has an effect of creatingthe nitrogen-enriched layer by combining with nitrogen. These effectsare obtained through V content of 0.01% or greater. On the other hand, Vcontent of greater than 0.50% reduces workability. Accordingly, in asituation in which V is contained in the steel, the V content is in arange of 0.01% to 0.50%. The V content is preferably in a range of 0.05%to 0.40%.

Nb: 0.01% to 0.15%

Nb combines with C and N contained in the steel and preventssensitization in the same way as Ti. Nb also has an effect of creatingthe nitrogen-enriched layer by combining with nitrogen. These effectsare obtained through Nb content of 0.01% or greater. On the other hand,Nb content of greater than 0.15% raises the recrystallizationtemperature such that an efficient high-speed pickling process such asdescribed in PTL 6 cannot be adopted. Accordingly, in a situation inwhich Nb is contained in the steel, the Nb content is in a range of0.01% to 0.15%. The Nb content is preferably in a range of 0.01% to0.10%.

Ca: 0.0003% to 0.0040%

Ca improves weldability by improving penetration of a welded part. Thiseffect is obtained through Ca content of 0.0003% or greater. However, Cacontent of greater than 0.0040% decreases corrosion resistance bycombining with S to form CaS. Accordingly, in a situation in which Ca iscontained in the steel, the Ca content is in a range of 0.0003% to0.0040%. The Ca content is preferably in a range of 0.0005% to 0.0030%.

B: 0.0003% to 0.0100%

B is an element that improves resistance to secondary workingbrittleness. This effect is exhibited when B content is 0.0003% orgreater. However, B content of greater than 0.0100% reduces ductilitydue to solid solution strengthening. Accordingly, in a situation inwhich B is contained in the steel, the B content is in a range of0.0003% to 0.0100%. The B content is preferably in a range of 0.0005% to0.0030%.

Through the above description, the chemical composition of the presentlydisclosed ferritic stainless steel has been explained.

In the chemical composition according to the present disclosure,components other than those listed above are Fe and incidentalimpurities.

In the presently disclosed ferritic stainless steel, it is vital thatthe chemical composition of the steel is appropriately controlled suchas to be in the range described above and that a nitrogen-enriched layersuch as described below is created in the surface layer part of thesteel by performing heat treatment in a controlled atmosphere prior tobrazing. Nitrogen concentration peak value at depth of within 0.05 μm ofsurface: 0.05 mass % to 0.30 mass %

In the presently disclosed ferritic stainless steel, a nitrogen-enrichedlayer is created that has a nitrogen concentration peak value of 0.05mass % to 0.30 mass % at a depth of within 0.05 μm of the surface of thesteel in a depth direction. This nitrogen-enriched layer can preventformation of a continuous and dense oxide film of Ti, Al, or the like atthe surface and, as a result, can improve brazing properties when aNi-containing brazing metal is used.

N in the nitrogen-enriched layer described above combines with Ti, Al,V, Nb, Cr, and the like in the steel. The following describes amechanism which the inventors consider to be responsible for thenitrogen-enriched layer inhibiting formation of a Ti or Al oxide film.

Specifically, formation of the nitrogen-enriched layer causes Ti and Alpresent in the surface layer part of the steel to combine with N suchthat the Ti and Al cannot diffuse to the surface of the steel.Furthermore, Ti and Al present inward of the nitrogen-enriched layercannot diffuse to the surface of the steel because the nitrogen-enrichedlayer acts as a barrier. According, formation of a Ti or Al oxide filmis inhibited as a result of Ti and Al in the steel not diffusing to thesurface.

Herein, formation of a Ti or Al oxide film at the surface cannot beadequately prevented if the nitrogen concentration peak value is lessthan 0.05 mass %. On the other hand, the surface layer part hardens ifthe nitrogen concentration peak value is greater than 0.30 mass %,making defects more likely to occur, such as fin plate cracking due tohot vibration of an engine or the like.

Therefore, the nitrogen concentration peak value at a depth of within0.05 μm of the surface has a value in a range of 0.05 mass % to 0.30mass %. The nitrogen concentration peak value is preferably in a rangeof 0.07 mass % to 0.20 mass %.

Note that the nitrogen concentration peak value at a depth of within0.05 μm of the surface referred to herein can for example be calculatedby measuring nitrogen concentration in the steel in a depth direction byglow discharge optical emission spectroscopy, dividing a maximum valuefor nitrogen concentration at a depth of within 0.05 μm of the steelsurface by a measured value for nitrogen concentration at a depth of0.50 μm, and multiplying the resultant value by the nitrogenconcentration of the steel obtained though chemical analysis.

Furthermore, the nitrogen-enriched layer described herein refers to aregion in which nitrogen is enriched due to permeation of nitrogen fromthe surface of the steel. The nitrogen-enriched layer is created in thesurface layer part of the steel and more specifically in a regionspanning for a depth of approximately 0.005 μm to 0.05 μm in the depthdirection from the surface of the steel.

The following describes a suitable production method for the presentlydisclosed ferritic stainless steel.

Molten steel having the chemical composition described above is preparedby steelmaking through a commonly known method such as using aconverter, an electric heating furnace, or a vacuum melting furnace, andis subjected to continuous casting or ingot casting and blooming toobtain a semi-finished casting product (slab).

The semi-finished casting product is hot rolled to obtain a hot-rolledsheet either directly without prior heating or after heating at 1100° C.to 1250° C. for 1 hour to 24 hours. The hot-rolled sheet is normallysubjected to hot band annealing at 800° C. to 1100° C. for 1 minute to10 minutes, but depending on the intended use, this hot band annealingmay be omitted.

Thereafter, the sheet is subjected to a combination of cold rolling andannealing to obtain a product steel sheet.

The cold rolling is preferably performed with a rolling reduction rateof 50% or greater in order to improve shape correction, extensibility,bendability, and press formability. Furthermore, the cold rolling andannealing process may be repeated two or more times.

Herein, it is necessary to create the above-described nitrogen-enrichedlayer in order to obtain the presently disclosed ferritic stainlesssteel. Treatment for creating the nitrogen-enriched layer is preferablyperformed during final annealing (finish annealing) carried out afterthe cold rolling.

Note that treatment for creating the nitrogen-enriched layer can beperformed in a separate step to annealing, such as, for example, after acomponent has been cut from the steel sheet. However, it is advantageousin terms of production efficiency to create the nitrogen-enriched layerduring the final annealing (finish annealing) carried out after the coldrolling because this allows the nitrogen-enriched layer to be createdwithout increasing the number of production steps.

The following describes conditions in treatment for creating thenitrogen-enriched layer.

Dew point: −20° C. or lower

If the dew point is higher than −20° C., a nitrogen-enriched layer isnot created because nitrogen from the surrounding atmosphere does notpermeate into the steel due to formation of an oxide film at the surfaceof the steel. Accordingly, the dew point is −20° C. or lower. The dewpoint is preferably −30° C. or lower.

Treatment atmosphere nitrogen concentration: 5 vol % or greater

If the nitrogen concentration of the treatment atmosphere is less than 5vol %, a nitrogen-enriched layer is not created because an insufficientamount of nitrogen permeates into the steel. Accordingly, the nitrogenconcentration of the treatment atmosphere is 5 vol % or greater. Thenitrogen concentration of the treatment atmosphere is preferably 10 vol% or greater. The remainder of the treatment atmosphere, besidesnitrogen, is preferably one or more selected from hydrogen, helium,argon, neon, CO, and CO₂.

Treatment temperature: 800° C. or higher

If the treatment temperature is lower than 800° C., a nitrogen-enrichedlayer is not created because nitrogen in the treatment atmosphere doesnot permeate into the steel. Accordingly, the treatment temperature is800° C. or higher. The treatment temperature is preferably 850° C. orhigher. However, the treatment temperature is preferably 1050° C. orlower because a treatment temperature of higher than 1050° C.(particularly 1100° C. or higher) leads to deformation of the steel. Thetreatment temperature is more preferably 1000° C. or lower, and isparticularly preferably 950° C. or lower.

The treatment time is preferably in the range of 5 seconds to 3600seconds. The reason for this is that nitrogen in the treatmentatmosphere does not sufficiently permeate into the steel if thetreatment time is shorter than 5 seconds, whereas the effects oftreatment reach saturation if the treatment time is longer than 3600seconds. The treatment time is preferably in a range of 30 seconds to300 seconds.

Through the above description, conditions in treatment for creating thenitrogen-enriched layer have been explained.

Although descaling may be performed after final annealing (finishannealing) by normal pickling or polishing, from a viewpoint ofproduction efficiency, it is preferable to perform descaling by adoptingthe high-speed pickling process described in PTL 6 in which mechanicalgrinding is performed using a brush roller, a polishing powder, shotblasting, or the like, and pickling is subsequently performed in anitrohydrochloric acid solution.

In a situation in which treatment for creating the nitrogen-enrichedlayer is performed during final annealing (finish annealing), careshould be taken to adjust the amount of pickling or polishing in orderthat the nitrogen-enriched layer that has been created is not removed.

EXAMPLES

Steels having the chemical compositions shown in Table 1 were eachprepared by steelmaking using a 50 kg small vacuum melting furnace. Eachresultant steel ingot was heated to 1150° C. in a furnace purged with Argas and was subsequently subjected to hot rolling to obtain a hot-rolledsheet having a thickness of 3.5 mm. Next, each of the hot-rolled sheetswas subjected to hot band annealing at 950° C. for 1 minute and shotblasting of the surface thereof with glass beads was performed.Thereafter, descaling was performed by carrying out pickling in whichthe sheet was immersed in a 200 g/1 sulfuric acid solution at atemperature of 80° C. for 120 seconds and was subsequently immersed in amixed acid of 150 g/1 of nitric acid and 30 g/1 of hydrofluoric acid ata temperature of 55° C. for 60 seconds.

Next, cold rolling was performed to reach a sheet thickness of 0.8 mmand annealing was performed under the conditions shown in Table 2 toobtain a cold-rolled and annealed sheet. Note that in a situation inwhich the external appearance of the sheet was deep yellow or blue, itwas judged that a thick oxide film had been formed and +20 A/dm²→−20A/dm² electrolytic picking was performed twice, with differentelectrolysis times, in a mixed acid solution of 150 g/1 of nitric acidand 5 g/1 of hydrochloric acid at a temperature of 55° C.

Evaluation of (1) ductility and measurement of (2) nitrogen-enrichedlayer nitrogen concentration were performed as described below for eachcold-rolled and annealed sheet obtained as described above.

Furthermore, brazing was carried out for each cold-rolled and annealedsheet using a Ni-containing brazing metal and the cold-rolled andannealed sheet was evaluated after brazing in terms of (3) corrosionresistance and (4) brazing properties. The evaluation of (4) brazingproperties was performed as described below for (a) joint gapinfiltration of the brazing metal and (b) joint strength of a brazedpart.

(1) Ductility evaluation

A JIS No. 13B tensile test piece was sampled at a right angle to therolling direction from each of the cold-rolled and annealed sheetsdescribed above, a tensile test was carried out in accordance with JIS Z2241, and ductility was evaluated using the following standard. Theevaluation results are shown in Table 2.

Good (pass): Elongation after fracture of 20% or greater

Poor (fail): Elongation after fracture of less than 20%

(2) Measurement of Nitrogen-Enriched Layer Nitrogen Concentration

The surface of each of the cold-rolled and annealed sheets was analyzedby glow discharge optical emission spectroscopy (hereinafter referred toas GDS). First, samples with different sputtering times from the surfacelayer were prepared and cross-sections thereof were observed by SEM inorder to prepare a calibration curve for a relationship betweensputtering time and depth.

Nitrogen concentration was measured while performing sputtering from thesurface of the steel to a depth of 0.50 μm. Herein, the measured valuesof Cr and Fe are fixed at the depth of 0.50 μm and thus a measured valuefor nitrogen concentration at the depth of 0.50 μm was taken to be thenitrogen concentration of the base material (steel substrate).

A highest peak value (greatest value) among measured nitrogenconcentration values within 0.05 μm of the steel surface was divided bythe measured nitrogen concentration value at the depth of 0.50 μm andthe resultant value was multiplied by a nitrogen concentration of thesteel obtained by chemical analysis to give a value that was taken to bea nitrogen concentration peak value at a depth of within 0.05 μm of thesurface. Nitrogen concentration peak values that were obtained are shownin Table 2.

(3) Evaluation of Corrosion Resistance

After brazing was carried out for each of the cold-rolled and annealedsheets, a 20 mm square test piece was sampled from a part to whichbrazing metal was not attached, and the test piece was covered by asealing material, but leaving a 11 mm square measurement surface.Thereafter, the test piece was immersed in a 3.5% NaCl solution at 30°C. and a corrosion resistance test was conducted in accordance with HS G0577 with the exception of the NaCl concentration. Pitting corrosionpotentials V_(c′100) that were measured are shown in Table 2.

When usage conditions of a heat exchanger part of an exhaust heatrecovery unit or an EGR cooler are taken into account, a pittingcorrosion potential V_(c′100) of 150 (mV vs SCE) or greater can bejudged to indicate excellent corrosion resistance.

(4) Evaluation of Brazing Properties

(a) Infiltration of Brazing Metal into Joint Gap

As illustrated in FIG. 1, a 30 mm square sheet and a 25 mm×30 mm sheetwere cut out from each of the cold-rolled and annealed sheets and thesetwo sheets were overlapped and clamped in place using a clamp jig with afixed torque force (170 kgf). Next, 1.2 g of a brazing metal was appliedonto an end surface of one of the sheets and brazing was carried out.After the brazing, the degree to which the brazing metal had infiltratedbetween the sheets was visually confirmed from a side surface part ofthe overlapped sheets and was evaluated using the following standard.The evaluation results are shown in Table 2. Note that in the drawings,the reference sign 1 indicates the cold-rolled and annealed sheet andthe reference sign 2 indicates the brazing metal.

Excellent (pass, particularly good): Brazing metal infiltration toopposite end relative to application end

Satisfactory (pass): Brazing metal infiltration over at least 50% andless than 100% of the overlapping length of the two sheets

Unsatisfactory (fail): Brazing metal infiltration over at least 10% andless than 50% of the overlapping length of the two sheets

Poor (fail): Brazing metal infiltration over less than 10% of theoverlapping length of the two sheets

(b) Joint Strength of Brazed Part

As illustrated in FIG. 2, portions of a JIS No. 13B tensile test piecethat had been split at the center thereof were overlapped by 5 mm andwere clamped in place using a clamp jig. Next, brazing was carried outby applying 0.1 g of a brazing metal to an overlapping part of one ofthe portions. After the brazing, a tensile test was conducted at normaltemperature and joint strength of the brazed part was evaluated usingthe following standard. The evaluation results are shown in Table 2.Note that in the drawings, reference sign 3 indicates the tensile testpiece.

Excellent (pass, particularly good): No brazed part fracture even at 95%or greater of tensile strength of base material (base material partfracture)

Satisfactory (pass): Brazed part fracture at 95% or greater of tensilestrength of base material

Unsatisfactory (fail): Brazed part fracture at 50% or greater and lessthan 95% of tensile strength of base material

Poor (fail): Brazed part fracture at less than 50% of tensile strengthof base material

In each evaluation of brazing properties described above, the brazingmetal was a representative Ni-containing brazing metal BNi-5 (19% Cr and10% Si in a Ni matrix) stipulated by Japanese Industrial Standards. Thebrazing was carried out in a sealed furnace. Furthermore, brazing wascarried out in a high vacuum atmosphere of 10⁻² Pa and was also carriedout in an Ar carrier gas atmosphere by enclosing Ar with a pressure of100 Pa after forming a high vacuum. A temperature pattern of the heattreatment involved performing treatment with a heating rate of 10° C/s,a first soaking time (step of equilibrating overall temperature) of 1800s at 1060° C., a heating rate of 10° C/s, and a second soaking time(step of actually carrying out brazing at a temperature equal to orhigher than the melting point of the brazing metal) of 600 s at 1170°C., followed by cooling of the furnace and purging of the furnace withexternal air (atmosphere) once the temperature had fallen to 200° C.

TABLE 1 Steel Chemical composition (mass %) symbol C Si Mn P S Cr Ti AlN Ni Mo Cu V Nb Ca B Remarks A 0.012 0.12 0.22 0.03 0.0011 21.5 0.2200.006 0.011 — — — — — — — Conforming steel B 0.010 0.09 0.18 0.02 0.001022.4 0.082 0.021 0.013 — 1.05 — — 0.125 — — Conforming steel C 0.0110.13 0.21 0.03 0.0013 21.5 0.124 0.041 0.010 — — — — — — — Conformingsteel D 0.009 0.20 0.19 0.03 0.0010 21.6 0.050 0.015 0.007 — — — — — — —Conforming steel E 0.015 0.20 0.21 0.04 0.0020 21.6 0.105 0.008 0.0120.12 — 0.45 0.201 0.125 0.0023 — Conforming steel F 0.008 0.22 0.22 0.030.0007 19.2 0.100 0.030 0.007 0.11 1.96 — 0.302 0.105 — 0.0004Conforming steel G 0.006 0.11 0.23 0.02 0.0010 16.5 0.066 0.035 0.0070.21 1.15 — 0.152 0.105 0.0020 — Conforming steel H 0.015 0.20 0.19 0.020.0010 21.7 0.102 0.005 0.013 0.19 — 0.48 0.225 0.085 — 0.0005Conforming steel I 0.007 0.10 0.22 0.03 0.0021 18.5 0.098 0.050 0.0130.18 — 0.49 0.223 0.095 — — Conforming steel J 0.008 0.26 0.21 0.030.0018 17.2 0.105 0.006 0.009 — — — 0.220 — 0.0032 0.0007 Conformingsteel K 0.007 0.23 0.22 0.02 0.0020 21.9 0.420 0.050 0.007 — — — — — — —Comparative steel L 0.012 0.22 0.13 0.03 0.0011 19.3 0.382 0.030 0.0160.09 1.86 0.42 — 0.192 — 0.0005 Comparative steel M 0.012 0.23 0.23 0.020.0010 21.5 0.041 0.015 0.014 0.21 — 0.44 0.162 0.008 — — Comparativesteel N 0.011 0.21 0.19 0.03 0.0016 21.5 0.105 0.070 0.008 0.15 — 0.510.124 0.089 — — Comparative steel O 0.007 0.21 0.19 0.03 0.0021 14.50.090 0.020 0.008 0.15 — — 0.094 0.068 — — Comparative steel

TABLE 2 Annealing conditions Measurement and evaluation results(nitrogen-enriched layer Nitrogen creation treatment conditions)concentration Pitting Atmosphere peak value of corrosion Dew TreatmentTreatment Post- nitrogen- potential Steel H₂ N₂ point temperature timeannealing Ductility enriched layer V

100 No symbol (vol %) (vol %) (° C.) (° C.) (s) pickling evaluation(mass %) (mV vs SCE) 1 A 5 95 −30 890 60 Yes Good 0.05 221 2 A 75 25 −55950 30 No Good 0.25 212 3 D 10 90 −45 890 90 Yes Good 0.10 208 4 C 20 80−25 860 60 Yes Good 0.08 215 5 B 75 25 −50 900 60 No Good 0.23 285 6 B 595 −35 890 30 Yes Good 0.08 292 7 E 80 20 −50 890 60 No Good 0.19 208 8F 75 25 −55 860 30 No Good 0.18 268 9 G 10 90 −35 880 60 Yes Good 0.06276 10 H 5 95 −30 880 30 Yes Good 0.08 211 11 I 30 70 −40 860 60 YesGood 0.08 192 12 J 10 90 −55 880 30 Yes Good 0.11 187 13 K 10 90 −45 95030 Yes Good 0.11 205 14 L 10 90 −30 890 30 Yes Poor 0.09 267 15 M 75 25−55 950 60 No Good 0.29 108 16 N 75 25 −55 890 60 Yes Good 0.22 212 17 O10 90 −40 890 30 Yes Good 0.10 87 18 A 10 90 −10 890 60 Yes Good 0.02211 19 A 100 0 −35 890 30 Yes Good 0.03 205 20 C 10 90 −45 750 60 YesPoor 0.03 199 Measurement and evaluation results Brazing propertiesevaluation Brazing properties evaluation (brazing in high vacuum)(brazing in Ar atmosphere) Brazing metal Brazed part Brazing metalBrazed part No infiltration joint strength infiltration joint strengthRemarks  1 Satisfactory Satisfactory Satisfactory Satisfactory Example 2 Satisfactory Excellent Satisfactory Satisfactory Example  3 ExcellentExcellent Excellent Excellent Example  4 Satisfactory SatisfactorySatisfactory Satisfactory Example  5 Excellent Satisfactory ExcellentSatisfactory Example  6 Satisfactory Excellent Excellent SatisfactoryExample  7 Excellent Excellent Excellent Satisfactory Example  8Excellent Excellent Excellent Satisfactory Example  9 ExcellentSatisfactory Satisfactory Satisfactory Example 10 Excellent SatisfactorySatisfactory Satisfactory Example 11 Excellent Satisfactory SatisfactorySatisfactory Example 12 Excellent Satisfactory Satisfactory SatisfactoryExample 13 Poor Unsatisfactory Poor Poor Comparative example 14Unsatisfactory Poor Poor Poor Comparative example 15 Excellent ExcellentExcellent Satisfactory Comparative example 16 Unsatisfactory Poor PoorPoor Comparative example 17 Excellent Satisfactory SatisfactorySatisfactory Comparative example 18 Poor Poor Poor Poor Comparativeexample 19 Poor Unsatisfactory Poor Poor Comparative example 20Unsatisfactory Poor Poor Poor Comparative example

indicates data missing or illegible when filed

Table 2 shows that for each of Examples 1-12, infiltration of thebrazing metal into the joint gap was good and joint strength of thebrazed part was good. Accordingly, it was demonstrated that Examples1-12 display good brazing properties even when a Ni-containing brazingmetal is used. Furthermore, Examples 1-12 had good corrosion resistanceand ductility.

In contrast, good brazing properties and/or good corrosion resistancewere not obtained in Comparative Examples 13-20 for which the chemicalcomposition or the nitrogen concentration peak value was outside of theappropriate range.

INDUSTRIAL APPLICABILITY

The present disclosure enables a ferritic stainless steel to be obtainedthat can be suitably used for heat exchanger components and the like ofexhaust heat recovery units and EGR coolers that are assembled bybrazing, and is therefore extremely useful in industry.

REFERENCE SIGNS LIST

1 cold-rolled and annealed sheet

2 brazing metal

3 tensile test piece

1. A ferritic stainless steel comprising a chemical compositioncontaining, in mass %: 0.003% to 0.020% of C; 0.05% to 1.00% of Si;0.10% to 0.50% of Mn; 0.05% or less of P; 0.01% or less of S; 16.0% to25.0% of Cr; 0.05% to 0.35% of Ti; 0.005% to 0.05% of Al; and 0.005% to0.025% of N, the balance being Fe and incidental impurities, wherein anitrogen-enriched layer is present that has a nitrogen concentrationpeak value of 0.05 mass % to 0.30 mass % at a depth of within 0.05 μm ofa surface of the steel.
 2. The ferritic stainless steel of claim 1,wherein the chemical composition further contains, in mass %, one ormore of: 0.05% to 0.50% of Ni; 0.10% to 3.00% of Mo; 0.10% to 0.60% ofCu; 0.01% to 0.50% of V; 0.01% to 0.15% of Nb; 0.0003% to 0.0040% of Ca;and 0.0003% to 0.0100% of B.
 3. A production method for the ferriticstainless steel of claim 1, comprising subjecting a slab having thechemical composition of claim 1 to hot rolling, subsequent hot bandannealing as required, and a subsequent combination of cold rolling andannealing to produce the ferritic stainless steel, wherein in finalannealing of the annealing, treatment for creating a nitrogen-enrichedlayer is performed at a temperature of 800° C. or higher in anatmosphere having a dew point of −20° C. or lower and a nitrogenconcentration of 5 vol % or greater.
 4. A production method for theferritic stainless steel of claim 2, comprising subjecting a slab havingthe chemical composition of claim 2 to hot rolling, subsequent hot bandannealing as required, and a subsequent combination of cold rolling andannealing to produce the ferritic stainless steel, wherein in finalannealing of the annealing, treatment for creating a nitrogen-enrichedlayer is performed at a temperature of 800° C. or higher in anatmosphere having a dew point of −20° C. or lower and a nitrogenconcentration of 5 vol % or greater.